Initial data commit

Task/Roman-numerals-Decode/JavaScript/roman-numerals-decode-8.js

@@ -0,0 +1,56 @@
+(() => {
+
+    // -------------- ROMAN NUMERALS DECODED ---------------
+
+    // Folding from right to left,
+    // lower leftward characters are subtracted,
+    // others are added.
+
+    // fromRoman :: String -> Int
+    const fromRoman = s =>
+        foldr(l => ([r, n]) => [
+            l,
+            l >= r ? (
+                n + l
+            ) : n - l
+        ])([0, 0])(
+            [...s].map(charVal)
+        )[1];
+
+    // charVal :: Char -> Maybe Int
+    const charVal = k => {
+        const v = {
+            I: 1,
+            V: 5,
+            X: 10,
+            L: 50,
+            C: 100,
+            D: 500,
+            M: 1000
+        } [k];
+        return v !== undefined ? v : 0;
+    };
+
+    // ----------------------- TEST ------------------------
+    const main = () => [
+            'MDCLXVI', 'MCMXC', 'MMVIII', 'MMXVI', 'MMXVII'
+        ]
+        .map(fromRoman)
+        .join('\n');
+
+
+    // ----------------- GENERIC FUNCTIONS -----------------
+
+    // foldr :: (a -> b -> b) -> b -> [a] -> b
+    const foldr = f =>
+        // Note that that the Haskell signature of foldr
+        // differs from that of foldl - the positions of
+        // accumulator and current value are reversed.
+        a => xs => [...xs].reduceRight(
+            (a, x) => f(x)(a),
+            a
+        );
+
+    // MAIN ---
+    return main();
+})();